1. Field of the Invention
The present invention relates to an image copying apparatus equipped with a scanner and a printer, a control method therefor, a program, and a method for generating a 3D-LUT.
2. Description of the Related Art
Image copying apparatuses are equipped with an input device, such as a scanner and an output device, such as a printer. Image copying apparatuses, for example, optically scan an original using a scanner, perform predetermined image processing on the obtained image data, and then output (print) the processed data using an output device.
Common input devices are those that irradiate an original with light using a fluorescent tube, an LED, or the like, and detect the light reflected from the original using a CCD. Common output devices are those that use a recording method such as an electrophotographic method, an ink jet method, etc.
Originals that have a broad color space, such as photographs, magazines, and the like, are often used. Moreover, input devices generally have a comparatively broad color gamut. On the other hand, although it does depend on the type of printing media used (e.g., glossy paper and photographic paper suitable for photographs, regular paper, which is most commonly used, and environmentally friendly recycled paper, among other such media), the color gamut of output devices is generally not as broad. In particular, there are many printing media, such as regular paper, that have very narrow color gamuts that can be output compared with the color gamuts of input devices.
What is necessary in such a case is a color space compression technique called gamut mapping. Gamut mapping determines how to express a color outside a color gamut, which cannot be output as-is, and furthermore, determines how to handle the color spaces within a color gamut. Methods thereof are generally roughly classified into two types: colorimetric (colorimetric matching method) and perceptual (perception-oriented method). The former gives priority to color reproducibility. Mapping is performed such that reproducible colors are expressed in colors identical to the original colors as much possible, and irreproducible colors are expressed in colors most similar to the original colors. In this case, there is a problem that while colorimetric matching is sufficiently achieved with respect to reproducible colors, irreproducible colors may be mapped on a single point, thereby impairing tonality. On the other hand, the latter method, or the perception-oriented method, achieves perceptually favorable color reproduction taking into consideration the overall balance so as to maintain tonality. However, since printed colors do not necessarily colorimetrically match the colors provided on the original, colors that should be reproducible sometimes do not match. Moreover, when there is a large difference in color gamut between the input and output devices, a printed result sometimes appears dull, with decreased saturation.
Japanese Patent Laid-Open No. H4-196675 discloses a technique related to gamut mapping which compresses colors presented on a display, having a broad color gamut, to fall within the color gamut of a printer. When a color space within a color gamut is compressed, this technique carries out colorimetric matching without compressing a portion within a specific region; with respect to the other portions, it carries out compression in a specific direction according to the internal ratio of the specific region to the maximum saturation/lightness. According to Japanese Patent Laid-Open No. H4-196675, gamut mapping that rectifies the aforementioned disadvantages of the colorimetric and perceptual methods, and performs natural color reproduction can be realized.
In addition, the subject matter of Japanese Patent Laid-Open No. 2001-28694 performs extension processing with respect to input data within the color gamut of an output device, so as to use the output gamut broadly, and performs gamut mapping with respect to input data outside the color gamut so as to sufficiently take advantage of the spaces within the color gamut, thereby enabling the color gamut of an output device to be used as broadly as possible.
Japanese Patent Laid-Open No. H4-217167 discloses three types of color processing—color processing using the isochromatic color reproduction system, color processing using the compressed color reproduction system, and color processing using the expanded color reproduction system—where any of these is selected according to the input original. At this time, the pixel value of an input image is accumulated, and if the accumulated value of output color reproduction pixels is not more than the predetermined value of all of the pixels, the color processing of the isochromatic color reproduction system is selected. Thereby, when copying is performed using a previously copied image as an original (hereinafter referred to as a “third-generation copy”), the colors of the original image and the printed image match one another closely.
However, although Japanese Patent Laid-Open No. H4-196675A reduces the generation of color dullness caused by decreased saturation, by providing a colorimetric matching region, a severe reduction in saturation cannot be prevented when the output color gamut is significantly smaller than the input color gamut. Therefore, it is difficult to obtain a preferable image with contrast.
In addition, although Japanese Patent Laid-Open No. 2001-28694 achieves favorable color reproduction by gamut mapping, which effectively uses the color space of the output color gamut, the reproducibility of the colors of the original image in the output image is poor. Therefore, when copying is performed in which faithful reproduction of a color is required, the image processing of Japanese Patent Laid-Open No. 2001-28694 is not preferable.
These problems are prominent in third-generation copying. With the subject matter of Japanese Patent Laid-Open No. H4-196675, the colors present in a compression region are further changed in third-generation copying, and the printed image appears even more different from the original image. The technique of Japanese Patent Laid-Open No. 2001-28694, too, cannot obtain a faithful copy due to overcoloring, or the like, caused by extension processing.
Although Japanese Patent Laid-Open No. H4-217167 takes third-generation copying into consideration, the input original has to be examined. Therefore, an operation called prescanning, by which an original is scanned prior to copying, is necessary, and memory space for storing the data of the entire scanned image is required. As a result, usability is impaired due to the long period of time required from when a request to start copying is made until the completion of copy processing. Moreover, installing a large-capacity memory for retaining the input image data increases the cost. Furthermore, information of the details of a plurality of processing procedures must be stored, and when a large-volume table is used in processing, a large ROM capacity is, therefore, required to store the same number of tables as there are processes.